1. Field of the Invention
The present invention pertains to integrated circuits and power supplies therefore. More particularly, the present invention pertains to the use of a power status signal to adjust the voltage supplied to and/or received by an integrated circuit or a portion thereof.
2. Description of Related Art
Improved power supplies for integrated circuits may ultimately improve integrated circuit manufacturing yield. The yield represents a percentage of integrated circuits produced which perform within the specified parameters, a percentage which may be improved if a power supply can limit the voltage supplied to the integrated circuit to a smaller operating range. The smaller operating range allows, for instance, the average or nominal voltage supply value to be set higher without causing the upper end of the operating range to exceed the highest allowable voltage for the integrated circuit (as determined by power constraints or otherwise).
Since transistors generally operate faster with a higher supply voltage, setting the nominal supply value higher may be quite advantageous. The result is that a larger percentage of integrated circuits will achieve target timing specifications due to the higher voltage. Thus, a more tightly controlled output voltage (i.e., a smaller output voltage range) may allow the integrated circuit to more easily meet timing specifications, thereby improving manufacturing yield.
A simple model of a prior art voltage supply includes a voltage source, an equivalent series inductance (ESL) in series with the voltage supply, and an equivalent series resistance (ESR) in series with a capacitor that is parallel to the voltage supply. The ESL and ESR components reflect the reality that no power supply can instantaneously respond to every possible change in load. When a dramatic decrease in current demand occurs, the ESL effectively continues driving current through the load, thus causing a voltage bounce. Similarly, when a large increase in current demand occurs, the ESR resists the immediate increase in current, causing a voltage droop. The output voltage returns to a steady state in a period determined mostly by the ESL and ESR values.
Coping with dramatic changes in current demand has become increasingly important in computer systems because various components may be shut down from time to time due to inactivity or for other reasons. For example, a particular unit of a processor or the entire processor may be shut down when not in use. Typically, a processor switches between high and low power consumption modes very quickly, perhaps in a single clock cycle. This dramatic and rapid change can produce the previously mentioned voltage bounce or droop.
These sudden voltage swings complicate the task of maintaining a supply voltage within a specified operating voltage for the component. The difficulty of maintaining a narrow voltage range is exacerbated by the increasing power consumption of such integrated circuits, and the larger current changes which occur when such integrated circuits switch between power consumption modes. Additionally, as new technology allows integrated circuits to operate at lower and lower voltages, the operating voltage range inevitably must shrink.
According to the prior art, a voltage supply may be kept within an acceptable range by utilizing components which provide a smaller ESL and a smaller ESR. This prior art technique which attempts to limit the ESL and ESR to produce a tighter voltage output range may prove inadequate to achieve a sufficiently narrow operating voltage in view of the larger abrupt current changes expected as integrated circuit technology advances. Moreover, even if a very narrow operating voltage range is not required to operate a particular circuit, it may be useful in improving yield and/or reducing power consumption. The prior art lacks an adequate mechanism to maintain a narrow supply voltage range for a circuit by considering the mode in which the circuit is operating.